mst CustomAction

I'm a bit of a newbie in this respect though i've used the standard msi offerings for deployment purposes for a while now. I've been given an MSI for a VPN solution that we need to implement across the board imminently. As part of the installation of this there are a number of switches that need activating, like to set the .ini configuration file location. I can see in InstEd that this command is specified in the CustomActions Folder in the MSI. my question is very simple....

could be I'm reading this wrong but why not just add the CONFIGURATIONFILE property in the MST and pass the value on the command line?
It's a public property so I wonder what the need is for a CA to populate it..